1. Field of the Invention
The present invention relates to a transmission apparatus based on the Synchronous Digital Hierarchy (SDH), and more particularly to an SDH transmission apparatus having a highly flexible system configuration.
2. Description of the Related Art
Recent advancement in international standardization of SDH transmission systems have accelerated the development of many types of standard multiplex transmission units, allowing a variety of digital transmission systems to be integrated in different ways. To meet the various needs, however, it is desired to develop such an SDH transmission apparatus that is equipped with a versatile mechanism to cross-connect low-order and high-order group transmission units, while allowing various classes of digital bitstreams to be handled with simple internal structure, regardless of their levels in the SDH multiplexing hierarchy.
Here, a few examples of conventional transmission apparatus are presented in FIGS. 30 to 32. FIG. 30 shows a transmission apparatus which has a cross-connect unit 101 to interconnect high-level group units including two STM-1 units 102 and 103, and low-order group units including a plurality of 2M units 104. The STM-1 units 102 and 103 are SDH transmission apparatus serving as multiplex/demultiplex facilities, which convert the signal frames from the Synchronous Transfer Module Level 1 (STM1) format to the Administrative Unit-4 (AU-4) format, and vice versa. They provide the cross-connect unit 101 with multiplexed signals having the AU-4 frame format. The 2M units 104 interface with lower speed services by acting as multiplex/demultiplex facilities which convert C-12 frames to Tributary Unit Group-2 (TUG-2) frames, and vice versa. Here, the term xe2x80x9cC-12xe2x80x9d denotes a container which accommodates 2.048 Mb/s Plesiochronous Digital Hierarchy (PDH) signals, where the name xe2x80x9c2M unitxe2x80x9d implies this specific bitrate to be handled. The 2M units 104 provides the cross-connect unit 101 with multiplexed signals having the TUG-2 frame format. The cross-connect unit 101 is a switching element that supports the consolidation, segregation, and grooming of paths running across a plurality of transmission lines. It also functions as a frame format converter that enables TUG-2 multiplexed signals sent from the plurality of 2M units 104 to be reformed into AU-4 multiplexed signals.
FIG. 31 is a block diagram of another type of a transmission apparatus, which employs a cross-connect unit 111 to interconnect high-level group units including two STM-1 units 112 and 113, and low-order group units including a plurality of 34M units 114. The STM-1 units 112 and 113 serve as multiplex/demultiplex facilities which convert STM-1 frames to AU-4 frames, and vice versa, thus providing the cross-connect unit 111 with AU-4 multiplexed signals. The 34M units 114 interface with lower speed services by acting as multiplex/demultiplex facilities to convert C-3 frames to TUG-3 frames, and vice versa. Here, the term xe2x80x9cC-3xe2x80x9d represents a container which accommodates 34.368 Mb/s PDH signals, as the name xe2x80x9c34M unitxe2x80x9d implies. The 34M units 114 supply the cross-connect unit 111 with TUG-3 multiplexed signals. Besides providing path switching capabilities, the cross-connect unit 111 serves as a frame format converter that allows TUG-3 multiplexed signals sent from the plurality of 34M units 114 to be reformed into AU-4 multiplexed signals.
FIG. 32 illustrates still another example of a transmission apparatus, which has a cross-connect unit 121 to interconnect high-level group units including two STM-4 units 122 and 123, and low-order group units including a plurality of 140M/STM-1 units 124. The STM-4 units 122 and 123 are multiplex/demultiplex facilities that convert STM-4 frames to AU-4 frames, and vice versa, thus providing the cross-connect unit 121 with AU-4 multiplexed signals. The 140M/STM-1 units 124 serve as multiplex/demultiplex facilities to convert C-4 or STM-1 frames to AU-4 frames, and vice versa. A C-4 frame is a container which accommodates 138.264 Mb/s PDH signals. The 140M/STM-1 units 114 supply the cross-connect unit 121 with AU-4 multiplexed signals. Note here that the cross-connect unit 121 does not have a function to convert signal formats, since its input and output signals have a unified format, AU-4.
As understood from FIGS. 30 to 32, conventional SDH transmission apparatus have different internal configurations depending on the hierarchical levels of multiplexed signals that they should deal with. Particularly, it is a problem that their integral cross-connect units have to be replaced with other ones, in order to introduce a different type of low-order group units into the system. Recall the aforementioned transmission apparatus of FIG. 30, for example. The cross-connect unit 101 used in this transmission apparatus is designed to interface with a particular type of low-order group units, i.e., 2M units. For this reason, it is not possible to install 34M units in place of the present 2M units 104. To do so, the cross-connect unit 101 must be changed to another type, such as a cross-connect unit 111 shown in FIG. 31. Similarly, since the cross-connect unit 111 shown in FIG. 31 cannot work with 140M/STM-1 units 124 in FIG. 32, one should replace it with another type of cross-connect unit, such as a cross-connect unit 121 shown in FIG. 32, in order to use 140M/STM-1 units.
The conventional transmission apparatus have still another problem as will be described below. FIG. 33 shows a system in which network devices 131 to 133 each consolidate 2.048 Mb/s or 34.368 Mb/s PDH signals into a single STM-1 bitstream and a network device 134 multiplexes four STM-1 bitstreams into a single STM-4 bitstream. Here, the network devices 131 to 133 each correspond to the transmission apparatus of FIG. 30 or FIG. 31, while the network device 134 corresponds to the transmission apparatus of FIG. 32. These four network devices 131 to 134 are under the control of a management unit 135 which monitors and supervises them through the connection paths indicated by the broken lines. As FIG. 33 shows, this conventional system is not simple since it requires two processing stages only to convert 2.048 Mb/s or 34.368 Mb/s PDH signals into a single STM-4 bitstream. As such, the increase in the number of network elements will make the management tasks more complex and result in a higher system cost that the customer should pay for.
Taking the above into consideration, an object of the present invention is to provide an SDH transmission apparatus with a simplified internal configuration, employing a versatile cross-connect unit that can deal with various classes of digital bitstreams regardless of their levels in the multiplexing hierarchy.
To accomplish the above object, according to the present invention, there is provided an SDH transmission apparatus which multiplexes transmission signals according to a multiplexing hierarchy. This apparatus comprises the following elements:
a mounting rack having a plurality of module slots and a backplane providing electrical interconnection among functional modules installed in the plurality of module slots;
a cross-connect unit, installed in dedicated cross-connect unit slots on the mounting rack, which comprises high-order group ports and low-order group ports;
at least one high-order group transmission unit, installed in dedicated high-order group slots on the mounting rack, which sends and receives multiplexed signals at a predetermined level of the multiplexing hierarchy to/from the high-order group ports of the cross-connect unit; and
a plurality of low-order group transmission units, installed in dedicated low-order group slots on the mounting rack, which sends and receives the multiplexed signals at the predetermined level of the multiplexing hierarchy to/from the low-order group ports of the cross-connect unit.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.